Triplet-triplet Energy Transfer Research Articles

Visible-Light Organic Photosensitizers Based on 2-(2-Aminophenyl)benzothiazoles for Photocycloaddition Reactions.

We have studied 2-(2-aminophenyl)benzothiazole and related derivatives for their photophysical properties in view of employing them as new and readily tunable organic photocatalysts. Their triplet energies were estimated by DFT calculations to be in the range of 52-57 kcal mol-1, suggesting their suitability for the [2+2] photocycloaddition of unsaturated acyl imidazoles with styrene derivatives. Experimental studies have shown that 2-(2-aminophenyl)benzothiazoles comprising alkylamino groups (NHMe, NHiPr) or the native amino group provide the best photocatalytic results in these visible-light mediated [2+2] reactions without the need of any additives, yielding a range of cyclobutane derivatives. A combined experimental and theoretical approach has provided insights into the underlying triplet-triplet energy transfer process.

Triplet Energy Gap-Regulated Room Temperature Phosphorescence in Host-Guest Doped Systems.

The organic room temperature phosphorescence (RTP) materials via host-guest doped method receive considerable attention in the fields of optoelectronics, bioimaging, and information encryption. Despite many host-guest doped materials with excellent RTP properties have been developed, their luminous mechanism is still limited. Here, a series of host-guest doped materials, using benzophenone as the host and quinone compounds as the guests, were constructed to investigate the effect of the triplet energy gap (ΔET) between the host and guest on triplet states population. The guest's triplet state is proposed to be a "triplet energy reservoir", gathering the triplet excitons to emit RTP when ΔET is large and returning triplet excitons to the host when ΔET is small. By combining the results of steady-state and delayed emission spectra, time-resolved transient absorption spectra, and theoretical calculations, a bidirectional energy transfer process is proved, which are triplet-triplet energy transfer and reverse triplet-triplet energy transfer processes. The thermal equilibrium of these two energy transfer processes can be regulated by the ΔET and temperature. The potential applications of these RTP properties are also realized in data encryption and anti-counterfeiting. This work provides valuable insight into the design of host-guest doped materials based on energy transfer mechanisms.

Visualizing Triplet Energy Transfer in Organic Near-Infrared Phosphorescent Host-Guest Materials.

Limited by the energy gap law, purely organic materials with efficient near-infrared room temperature phosphorescence are rare and difficult to achieve. Additionally, the exciton transition process among different emitting species in host-guest phosphorescent materials remains elusive, presenting a significant academic challenge. Herein, using a modular nonbonding orbital-π bridge-nonbonding orbital (n-π-n) molecular design strategy, we develop a series of heavy atom-free phosphors. Systematic modification of the π-conjugated cores enables the construction of a library with tunable near-infrared phosphorescence from 655 to 710 nm. These phosphors exhibit excellent performance under ambient conditions when dispersed into a 4-bromobenzophenone host matrix, achieving an extended lifetime of 11.25 ms and a maximum phosphorescence efficiency of 4.2 %. Notably, by eliminating the interference from host phosphorescence, the exciton transition process in hybrid materials can be visualized under various excitation conditions. Spectroscopic analysis reveals that the improved phosphorescent performance of the guest originates from the triplet-triplet energy transfer of abundant triplet excitons generated independently by the host, rather than from enhanced intersystem crossing efficiency between the guest singlet state and the host triplet state. The findings provide in-depth insights into constructing novel near-infrared phosphors and exploring emission mechanisms of host-guest materials.

Sergeant-and-Soldier Effect in an Organic Room-Temperature Phosphorescent Host-Guest System.

Host-guest systems have emerged as an efficient strategy for promoting organic room temperature phosphorescence (RTP). Despite the advantages of doping guest molecules into a host matrix, the complexity of these systems and the lack of techniques to visualize host-guest interactions at the molecular scale pose significant challenges in understanding the underlying mechanisms. Here, a novel host-guest RTP system is developed by incorporating low concentrations (1-10 mol%) of TPP-4C-BI (guest) into crystalline TPP-4C-Cz (host). Utilizing structural isomerism, the guest molecules are regularly incorporated into the host crystal lattice, resulting in phosphorescence quantum yields almost ten times higher than the pure compounds. The system enabled resolution of the molecular packing of the single crystal through X-ray diffraction, providing unprecedented visualization of host-guest interactions. A "sergeant-and-soldier" effect, where the minority dopant molecules (sergeants) significantly influence the packing arrangement of the host molecules (soldiers), enhances RTP is identified. Further analyses revealed that due to the host molecule's inefficient phosphorescence pathway, its long-lived dark triplets are channeled to the guest via triplet-triplet energy transfer (TTET), allowing the excited energy to radiatively decay more efficiently. These insights advance the understanding of RTP mechanisms and offer practical implications for designing high-efficiency phosphorescent materials.

Visible and near-infrared light-induced photoclick reactions.

Photoclick reactions combine the advantages offered by light-driven processes, that is, non-invasive and high spatiotemporal control, with classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photocrosslinking, protein labelling and bioimaging. Despite these advances, most photoclick reactions typically require near-ultraviolet (UV) and mid-UV light to proceed. UV light can trigger undesirable responses, including cellular apoptosis, and therefore, visible and near-infrared light-induced photoclick reaction systems are highly desirable. Shifting to a longer wavelength can also reduce degradation of the photoclick reagents and products. Several strategies have been used to induce a bathochromic shift in the wavelength of irradiation-initiating photoclick reactions. For instance, the extension of the conjugated π-system, triplet-triplet energy transfer, multi-photon excitation, upconversion technology, photocatalytic and photoinitiation approaches, and designs involving photocages have all been used to achieve this goal. Current design strategies, recent advances and the outlook for long wavelength-driven photoclick reactions are presented.

Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions.

Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. In this report, we show that modification of the X counterions constitutive of [Ru(bpy)3](X)2 photocatalysts modulates their catalytic activities in intermolecular [2 + 2] cycloaddition reactions operating through triplet-triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited-state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BArF 4 -) or a tightly bound (TsO-) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)3](BArF 4)2 exhibiting the shortest 3MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to >1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.

Long Excited-State Lifetimes in Three-Coordinate Copper(I) Complexes via Triplet-Triplet Energy Transfer to Pyrene-Decorated Isocyanides.

There has been much effort to improve excited-state lifetimes in photosensitizers based on earth-abundant first-row transition metals. Copper(I) complexes have gained significant attention in this field, and in most cases, sterically driven approaches are used to optimize their lifetimes. This study presents a series of three-coordinate copper(I) complexes (Cu1-Cu3) where the excited-state lifetime is extended by triplet-triplet energy transfer. The heteroleptic compounds feature a cyclohexyl-substituted β-diketiminate (CyNacNacMe) paired with aryl isocyanide ligands, giving the general formula Cu(CyNacNacMe)(CN-Ar) (CN-dmp = 2,6-dimethylphenyl isocyanide for Cu1; CN-pyr = 1-pyrenyl isocyanide for Cu2; CN-dmp-pyr = 2,6-dimethyl-4-(1-pyrenyl)phenyl isocyanide for Cu3). The nature, energies, and dynamics of the low-energy triplet excited states are assessed with a combination of photoluminescence measurements at room temperature and 77 K, ultrafast transient absorption (UFTA) spectroscopy, and DFT calculations. The complexes with the pyrene-decorated isocyanides (Cu2 and Cu3) exhibit extended excited-state lifetimes resulting from triplet-triplet energy transfer (TTET) between the short-lived charge-transfer excited state (3CT) and the long-lived pyrene-centered triplet state (3pyr). This TTET process is irreversible in Cu3, producing exclusively the 3pyr state, and in Cu2, the 3CT and 3pyr states are nearly isoenergetic, enabling reversible TTET and long-lived 3CT luminescence. The improved photophysical properties in Cu2 and Cu3 result in improvements in activity for both photocatalytic stilbene E/Z isomerization via triplet energy transfer and photoredox transformations involving hydrodebromination and C-O bond activation. These results illustrate that the extended excited-state lifetimes achieved through TTET result in newly conceived photosynthetically relevant earth-abundant transition metal complexes.

Naphthyl substituted guest induce efficient room temperature phosphorescence by a triplet-triplet energy transfer mechanism

Room-temperature phosphorescent (RTP) materials have attracted more and more attention due to their long luminescence life and large Stokes displacement. Host-guest doping systems are highly regarded for their simplicity of preparation and the absence of complex synthesis processes. Strong RTP emission can be activated by the doping of a guest molecules, in particular by substituting the phenyl group in the host structure with a naphthalene group. In this study, derivatives of benzophenone were constructed as the host molecule, and used 2-Benzoylnaphthalene, a commercially available naphthyl-substituted analogue of benzophenone, as the guest molecule. The phosphorescent lifetime of the doped system can reach 335 ms and the phosphorescent quantum yield can reach 58 %, which is expected to be used in anti-counterfeiting encryption, biological imaging and other fields. This work provides a general and effective host-guest doping strategy for constructing various organic room temperature phosphorescent materials.

Triplet-Triplet Annihilation Upconverting Liposomes: Mechanistic Insights into the Role of Membranes in Two-Dimensional TTA-UC.

Triplet-triplet annihilation upconversion (TTA-UC) implemented in nanoparticle assemblies is of emerging interest in biomedical applications, including in drug delivery and imaging. As it is a bimolecular process, ensuring sufficient mobility of the sensitizer and annihilator to facilitate effective collision in the nanoparticle is key. Liposomes can provide the benefits of two-dimensional confinement and condensed concentration of the sensitizer and annihilator along with superior fluidity compared to other nanoparticle assemblies. They are also biocompatible and widely applied across drug delivery modalities. However, there are relatively few liposomal TTA-UC systems reported to date, so systematic studies of the influence of the liposomal environment on TTA-UC are currently lacking. Here, we report the first example of a BODIPY-based sensitizer TTA-UC system within liposomes and use this system to study TTA-UC generation and compare the relative intensity of the anti-Stokes signal for this system as a function of liposome composition and membrane fluidity. We report for the first time on time-resolved spectroscopic studies of TTA-UC in membranes. Nanosecond transient absorption data reveal the BODIPY-perylene dyad sensitizer has a long triplet lifetime in liposome with contributions from three triplet excited states, whose lifetimes are reduced upon coinclusion of the annihilator due to triplet-triplet energy transfer, to a greater extent than in solution. This indicates triplet energy transfer between the sensitizer and the annihilator is enhanced in the membrane system. Molecular dynamics simulations of the sensitizer and annihilator TTA collision complex are modeled in the membrane and confirm the co-orientation of the pair within the membrane structure and that the persistence time of the bound complex exceeds the TTA kinetics. Modeling also reliably predicted the diffusion coefficient for the sensitizer which matches closely with the experimental values from fluorescence correlation spectroscopy. The relative intensity of the TTA-UC output across nine liposomal systems of different lipid compositions was explored to examine the influence of membrane viscosity on upconversion (UC). UC showed the highest relative intensity for the most fluidic membranes and the weakest intensity for highly viscous membrane compositions, including a phase separation membrane. Overall, our study reveals that the co-orientation of the UC pair within the membrane is crucial for effective TTA-UC within a biomembrane and that the intensity of the TTA-UC output can be tuned in liposomal nanoparticles by modifying the phase and fluidity of the liposome. These new insights will aid in the design of liposomal TTA-UC systems for biomedical applications.

Organo-Photocatalytic Anti-Markovnikov Hydroamidation of Alkenes with Sulfonyl Azides: A Combined Experimental and Computational Study.

The construction of C(sp3)-N bonds via direct N-centered radical addition with olefins under benign conditions is a desirable but challenging strategy. Herein, we describe an organo-photocatalytic approach to achieve anti-Markovnikov alkene hydroamidation with sulfonyl azides in a highly efficient manner under transition-metal-free and mild conditions. A broad range of substrates, including both activated and unactivated alkenes, are suitable for this protocol, providing a convenient and practical method to construct sulfonylamide derivatives. A synergistic experimental and computational mechanistic study suggests that the additive, Hantzsch ester (HE), might undergo a triplet-triplet energy transfer manner to achieve photosensitization by the organo-photocatalyst under visible light irradiation. Next, the resulted triplet excited state 3HE* could lead to a homolytic cleavage of C4-H bond, which triggers a straightforward H-atom transfer (HAT) style in converting sulfonyl azide to the corresponding key amidyl radical. Subsequently, the addition of the amidyl radical to alkene followed by HAT from p-toluenethiol could proceed to afford the desired anti-Markovnikov hydroamidation product. It is worth noting that mechanistic pathway bifurcation could be possible for this reaction. A feasible radical chain propagation mechanistic pathway is also proposed to rationalize the high efficiency of this reaction.

Rapid room-temperature phosphorescence chiral recognition of natural amino acids

Chiral recognition of amino acids is very important in both chemical and life sciences. Although chiral recognition with luminescence has many advantages such as being inexpensive, it is usually slow and lacks generality as the recognition module relies on structural complementarity. Here, we show that one single molecular-solid sensor, L-phenylalanine derived benzamide, can manifest the structural difference between the natural, left-handed amino acid and its right-handed counterpart via the difference of room-temperature phosphorescence (RTP) irrespective of the specific chemical structure. To realize rapid and reliable sensing, the doped samples are obtained as nanocrystals from evaporation of the tetrahydrofuran solutions, which allows for efficient triplet-triplet energy transfer to the chiral analytes generated in situ from chiral amino acids. The results show that L-analytes induce strong RTP, whereas the unnatural D-analytes produce barely any afterglow. The method expands the scope of luminescence chiral sensing by lessening the requirement for specific molecular structures.

Triplet-Triplet Energy Transfer: A Simple Strategy for an Efficient Visible Light-Induced Photoclick Reaction.

Photoclick reactions combine the advantages offered by light-driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo-crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet-triplet energy transfer as a fast and selective way to enable visible light-induced photoclick reactions. Specifically, we show that 9,10-phenanthrenequinones (PQs) can efficiently react with electron-rich alkenes (ERAs) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ-ERAs system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light-induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light.

Triplet Energy Transfer Mechanism in Copper Photocatalytic N- and O-Methylation.

Methylation reactions are chemically simple but challenging to perform under mild and non-toxic conditions. A photochemical energy transfer strategy was merged with copper catalysis to enable fast reaction times of minutes and broad applicability to N-heterocycles, (hetero-)aromatic carboxylic acids, and drug-like molecules in high yields and good functional group tolerance. Detailed mechanistic investigations, using kinetic analysis, aprotic MS, UV/Vis, and luminescence quenching experiments revealed a triplet-triplet energy transfer mechanism between hypervalent iodine(III) reagents and readily available photosensitizers.

Photochemical processes to cellular DNA damage by UV radiation of different wavelengths: biological consequences

UV radiation of sunlight induces in cellular DNA of different organisms photochemical reactions, which may lead to the development of series biological responses to arising lesions, including apoptosis, mutagenesis, and carcinogenesis. The chemical nature and the amount of DNA lesions depend on the wavelength of UV radiation. Photons of UV radiation in the region B (UVB, 290–320 nm) cause the production of two main defects, namely, cyclobutane pyrimidine dimers and, with a less yield, pyrimidine (6-4) pyrimidone photoproducts; their formation is the result of the direct UVB photon absorption by DNA bases. Photons of UV radiation in the region A (UVA, 320–400 nm) induce only cyclobutane dimers that can be formed by triplet-triplet energy transfer from cellular chromophores, absorbing photons of this UV region, to DNA thymine bases. UVA is much more effective than UVB in the sensitized oxidatively lesion formation in DNA such as single strand breaks and oxidized bases; among those, 8-oxo-dihydroguanine is the most frequent since it can be produced from several oxidation processes. In recent years, multiple papers, reporting novel, more detailed information about molecular mechanisms of photochemical reactions underlying the formation of different lesions in DNA were published. The present review mainly aims at summarizing and analyzing data contained in these publications, particularly regarding oxidative reactions that are initiated by reactive oxygen species and radicals generated by potential endogenous photosensitizers such as pterins, riboflavin, protoporphyrin IX, NADH, and melanin. The role of specific DNA photoproducts in genotoxic processes induced in living systems by UV radiation of different wavelengths, including human skin carcinogenesis, is discussed.

Site-Selective Photo-Crosslinking of Stilbene Pairs in a DNA Duplex Mediated by Ruthenium Photocatalyst.

We herein report a method for site-selective photo-crosslinking of a DNA duplex. A stilbene pair was introduced into a DNA duplex and a ruthenium complex was conjugated with a triplex-forming oligonucleotide. We demonstrated that [2+2] photocycloaddition of the stilbene pair occurred upon irradiation with visible light when the ruthenium complex was in close proximity due to triplex formation. No reaction occurred when the ruthenium complex was not in proximity to the stilbene pair. The wavelength of visible light used was of lower energy than the wavelength of UV light necessary for direct excitation of stilbene. Quantum chemical calculation indicated that ruthenium complex catalyzed the photocycloaddition via triplet-triplet energy transfer. Site selectivity of this photo-crosslinking system was evaluated using a DNA duplex bearing two stilbene pairs as a substrate; we showed that the site of crosslinking was precisely regulated by the sequence of the oligonucleotide linked to the ruthenium complex. Since this method does not require orthogonal photoresponsive molecules, it will be useful in construction of complex photoresponsive DNA circuits, nanodevices and biological tools.

Tunable Phosphorescence in Metastable CuI Assemblies Prepared by Acid‐Fueled Crystal‐to‐Crystal Conversion

AbstractAcid acts as a “fuel” in driving kinetic self‐assembly, with crystals of the CuII complex ([CuIICl2(BH)2], BH=benzothiadiazole helicene derivative) being successfully converted to crystals of 1 ([H2BH][CuBr3]) and 2 ([H2BH][CuBr3⋅2H2O]). The metastable features of 1 and 2 enable CuI guest release, with transformation into a thermodynamically favorable form 3 (H2BH⋅2Br). Compound 1 exhibits dual phosphorescence at 452 nm (3MC) and 709 nm (3CT) at room temperature. At lower temperatures, the 3CT band disappears and the 3MC band weakens, while a new emission band at 621 nm (3ππ*) is enhanced. In 1, the subtle structural change and significant emission change depending on temperature is reversible. In contrast, 2 exhibits only 3MC emission at room temperature, with less efficient triplet‐triplet energy transfer (TTET) below 250 K, and with residual 3ππ* emission occurring upon returning to room temperature, even after one week.

Amplifying dual-visible-light photoswitching in aqueous media via confinement promoted triplet-triplet energy transfer.

Achieving visible-light photochromism is a long-term goal of chemists keen to exploit the opportunities of molecular photoswitches in multi-disciplinary research studies. Triplet-sensitization offers a flexible approach to building diverse visible-light photoswitches using existing photochromic scaffolds, circumventing the need for sophisticated molecular design and synthesis. Unfortunately, distance-dependence and environment-sensitivity of triplet-excited species remain as key challenges that severely impair sensitization efficiency and limit their practical availability. We present herein a nature-inspired nanoconfinement strategy in which a triplet-sensitized visible-light photoswitch/sensitizer system is assembled into nanoconfined micelles (d ∼ 40 nm). A ca. 10-fold efficiency increase of triplet-triplet energy transfer for photochromism as well as an amplified fluorescence on/off contrast upon bi-directional visible-light excitation (470/560 nm) was achieved in full aqueous media. By virtue of this, the hybrid photoswitchable system is successfully applied for both flash information encryption and multiple dynamic cell imaging assays, further proving its versatility in materials and life science.

Energy transfer photocatalysis: exciting modes of reactivity.

Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloadditions, selective homolytic bond cleavages and strain-release chemistry, isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after organic motifs in a mild, selective, and sustainable manner. Building on the photophysical foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.

Visible-Light-Activated Carbon Monoxide Release from Porphyrin-Flavonol Hybrids.

We report on porphyrin-flavonol hybrids consisting of a porphyrin antenna and four covalently bound 3-hydroxyflavone (flavonol) groups, which act as highly efficient photoactivatable carbon monoxide (CO)-releasing molecules (photoCORMs). These bichromophoric systems enable activation of the UV-absorbing flavonol chromophore by visible light up to 650 nm and offer precise spatial and temporal control of CO administration. The physicochemical properties of the porphyrin antenna system can also be tuned by inserting a metal cation. Our computational study revealed that the process occurs via endergonic triplet-triplet energy transfer from porphyrin to flavonol and may become feasible thanks to flavonol energy stabilization upon intramolecular proton transfer. This mechanism was also indirectly supported by steady-state and transient absorption spectroscopy techniques. Additionally, the porphyrin-flavonol hybrids were found to be biologically benign. With four flavonol CO donors attached to a single porphyrin chromophore, high CO release yields, excellent uncaging cross sections, low toxicity, and CO therapeutic properties, these photoCORMs offer exceptional potential for their further development and future biological and medical applications.

Triplet-triplet energy transfer between host and guest induced strong phosphorescence in the organic doped system

Recently, purely organic room-temperature phosphorescence (RTP) from guest-host-doped systems has grown exponentially due to their high modulation flexibility. Benzophenone has been found as a preferred host matrix thanks to its low melting point, minimal biological toxicity, remarkable stability, and cost-effectiveness. However, the precise role of benzophenone remains elusive and has been debated within the scientific community concerning mechanistic elucidations. Here, five indole derivatives were designed and synthesized as guest molecules while retaining benzophenone as the host to construct doped materials exhibiting outstanding RTP properties. Our experimental findings unequivocally establish that the host benzophenone initially absorbs excitation energy, subsequently intersystem crossing into triplet exciton, facilitating the exciton transfer to guest molecule through the triplet energy transfer and culminating in guest RTP emission. Mechanistic investigation corroborates the superiority of benzophenone in possessing a large spin-orbit coupling constant and quantitatively triplet energy transfer. Collectively, this work provides compelling evidence deciphering energy transfer between host and guest entities for the manifestation of phosphorescence performance in benzophenone-hosted doped materials.